Through fossil-fuel combustion, commercial fertilizer production, and legume and rice cultivation, humans are mobilizing nitrogen (N) at scales that exceed natural terrestrial rates. Globally, approximate to 140 Tg of reactive N are mobilized each year; activities in the People's Republic of China (China) and the United States account for approximately 30% of this reactive N. This paper compares N mobilization rates and nitrogen oxides (NOx) and ammonia (NH3) atmospheric emission rates for China and the United States over the last few decades and projects the rates to the year 2020. We have estimated that the United States and China currently mobilize about 20 and 25 Tg N yr(-1), respectively. In the United States, commercial-fertilizer use accounts for about 50% of the annual total; in China, it accounts for about 80%. N emissions to the United States atmosphere are dominated by NOx. China's N emissions are dominated by NH3 from fertilizer use and domestic animal waste. If recent trends continue, we have projected that by 2020 N mobilization in the United States will differ little from current levels although N mobilization in China will increase significantly because of increased fertilizer use and fossil-fuel combustion.
